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The Humvee vehicles used by New Zealand soldiers killed in Afghanistan were all but retired by US forces several years ago because of their vulnerability to roadside bomb attacks.

Defence Force chief Lieutenant General Rhys Jones said yesterday no vehicle would have prevented the three deaths in Bamiyan, and that the Humvee was the only land vehicle in service that could navigate the rough road the patrol was travelling on.

However, coalition forces including those from the United States, Britain and Australia have been pushing through major upgrades to their patrol vehicles since improvised explosive devices (IEDs) became the weapon of choice of insurgents.

The New Zealand Provincial Reconstruction Team (PRT) depended on Light Armoured Vehicles (LAVs) and Humvees for patrols in Bamiyan - neither of which had significant protection against roadside bombs.

The United States military changed its policy on using Humvees for patrols as the number of reported IED incidents in Afghanistan rose from 81 in 2003 to 7228 in 2009. More than three years ago, USA Today reported that US soldiers needed approval of senior commanders to patrol in a Humvee.

The vehicle's fatal flaw, a 2008 Pentagon inspector-general's report found, was that its "flat bottom, low weight, low ground clearance and aluminum body" left it vulnerable to IEDs.

In April this year, US Army Major General Tony Cucolo told the US National defence magazine: "For combat, we know the Humvee is no longer feasible." It was used only as a support vehicle by US troops.

In the mid-2000s, the Pentagon began introducing MRAPs - Mine Resistant Ambush Protected vehicles - which were more resistant to IED attacks thanks to a higher chassis, sloped hull and blast-absorption technology.

According to the US Defence Department, the casualty rate for an MRAP in IED attacks was 6 per cent. In contrast, a Humvee's casualty rate was 22 per cent.

The use of MRAPs reduced troop deaths from 76 in July 2009 to 57 in July 2010 despite there being significantly more IED strikes in the latter year, according to International Security Assistance Force commanders.

The new vehicles introduced by coalition forces to defend against IEDs have two key traits - a V-shaped hull to deflect blasts and the ability to keep driving after a wheel has been blown off.

The Australian Defence Force, like New Zealand, uses Humvees and LAVs in Afghanistan. But it also has a stronger vehicle in the Australian-made Bushmaster, a 12-tonne truck which has a sloped hull to protect against roadside bomb explosions.

Australia has lost 33 soldiers in Afghanistan, 14 of them to IED attacks. But no Australian has been killed in a Bushmaster.

The ADF is also ordering new Hawkei vehicles which have a protective, V-shaped hull and an outer skin which absorbs some of the force of an explosion.

The British forces retired the old "snatch" Land Rover patrol vehicles after an outcry over the increasing numbers of deaths caused by IED strikes.

WHO HAS WHAT FOR COMBAT* United StatesIn use: Cougar mine resistant vehiclesGone: Humvee